The present invention relates to a method of separating 2,6-dimethylnaphthalene from a feed material containing a mixture of dimethylnaphthalene isomers (the term "dimethylnaphthalene" is hereinafter referred to as DMN). More particularly, the present invention relates to a method of selective adsorption and desorption of 2,6-DMN which employs a specified solid adsorbent in combination with a specified desorbent followed by crystallization with said desorbent in order to separate 2,6-DMN from a stream of feed material containing 2,6-DMN and at least one other DMN isomer.
A purity of 2,6-DMN hereinafter mentioned means a value obtained by dividing a weight of 2,6-DMN in a unit volume by a weight of feed material containing no desorbent in the unit volume.
It has been recognized in the prior art that a zeolite X or Y containing a certain species of cations on ion-exchangeable cation sites can be employed to separate a particular DMN isomer from a mixture containing other isomers. See, for example, U.S. Pat. Nos. 3,133,126 and 3,114,782 which show that a zeolite X containing sodium or calcium on ion-exchangeable cation sites is useful as an adsorbent for effectively selective adsorption of one DMN isomer from another.
Japanese patent publication No. 945/1977 shows that 2,7-DMN can be selectively separated from an eutectic mixture of 2,6-DMN and 2,7-DMN using zeolite Y, with benzene, toluene or ortho xylene being used as a desorbent.
Japanese patent publication No. 27578/1974 shows that 2,6-DMN can be separated from other isomers using zeolite Y. Dutch Pat. No. 7307794, U.S. Pat. Nos. 3,772,399, 3,840,610, 3,895,080, 4,014,949, etc., show that zeolite Y is useful as an adsorbent in separating cyclic hydrocarbons.
However, concerning the selective adsorption of one DMN isomer from another, U.S. Pat. Nos. 3,133,126 and 3,114,782 merely present a batch method for dilute solution of a mixture of DMN isomers in a container in the presence of paraffin which is not substantially involved in the actual case of adsorption. These patents do not make any mention of the participation of desorbents which are indispensable to commercial operations of selective adsorption process, such as continuous separation of DMN isomers using simulated moving beds, etc.
The disclosure of desorbents which are necessary for commercial operations of selective adsorption of DMN isomers also lacks from Japanese patent publication No. 27578/1974, U.S. Pat. Nos. 3,772,399, 3,895,080, 4,014,949, and Dutch Pat. No. 7307794. In general, in order to ensure successful operations of adsorption separation, the system to be employed must satisfy the following requirements for an adsorbent and a desorbent: as for the adsorbent, it must offer a high separation factor, causes no deterioration of the substances to be processed, and allows for rapid adsorption and desorption of said substances; as for the desorbent, it must be capable of promoting the adsorption and desorption of the substances being processed. If these requirements are not satisfied, problems such as increased tailings of the processed substances occur and the substance of interest cannot be separated with high efficiency.
The technique shown in Japanese patent publication No. 945/1977 is somewhat advanced in that it proposes the use of an adsorbent in combination with a desorbent. However, this reference suggests nothing about a specific method for selectively separating a desired isomer of high purity from the eutectic mixture obtained.
The following methods could be employed to further enrich a certain DMN isomer which is obtained by adsorption separation:
(1) the mixture is added in an appropriate solvent and the solution is repeatedly subjected to the same operations of adsorption and desorption;
(2) the mixture is crystallized in a lower aliphatic alcohol such as methanol in accordance with the method disclosed in Japanese patent publication No. 24505/1976 or 34695/1972; and
(3) the mixture is subjected to "solvent-free crystallization" in accordance with the method disclosed in Japanese patent publication No. 27578/1974.
However, the first of these methods has the disadvantage that since impurities having aromatic rings that are inseparable from 2,6-DMN by this method are present in small amount in the feed, it is impossible to separate 2,6-DMN in a purity of, for example, 99% and higher, even if repeated cycles of adsorption and desorption are performed. This method is also economically disadvantageous and is not suitable for use in commercial operations since it requires a large quantity of desorbent (a desorbent is necessary for each cycle of adsorption and desorption) and because the used desorbent must be recovered from the system in subsequent processing. The second method is not necessarily advantageous, either, because the solubility of DMN isomers in lower aliphatic alcohols such as methanol is so low that when the raffinate, or residual components other than 2,6-DMN that are present in the mother liquor for crystallization, is transferred to a subsequent step, a large quantity of a lower aliphatic alcohol must be accompanied. The third method is capable of yielding a desired substance in high purity but this involves eutectic problems and hence is not advantageous with respect to high recovery. Furthermore, this is not adapted to continuous operations and hence is not advantageous for commercial application.
The present inventors conducted intensive studies in order to solve the aforementioned problems of the prior art techniques and found that when a zeolite Y containing specific metallic ions was used as an adsorbent in combination with a specified desorbent, efficient adsorption separation of 2,6-DMN could be accomplished. The present inventors also found that 2,6-DMN of high purity could be obtained in high yield by subsequently performing crystallization under specified conditions using said desorbent. The present invention has been accomplished on the basis of these findings.